1. Field of the Invention
The present invention relates to a magnetic disk apparatus having magnetic disks and magnetic heads provided within its enclosure, and, more particularly, to a magnetic disk apparatus which protects the magnetic disks and magnetic heads from an external electromagnetic field.
2. Description of the Related Art
A magnetic disk apparatus has rotatable magnetic disks and magnetic heads for writing and reading information on and from the magnetic disks. Each magnetic head is positioned over a desired track position of the associated magnetic disk to read data from that track or write data thereon.
This magnetic disk apparatus is widely used as an external storage device. To meet the recent demand of downsizing of computer systems, particularly, the use of such a magnetic disk apparatus in a notebook type computer, a word processor, a portable, palmtop computer and so forth becomes popular. There is therefore a demand for small and slim magnetic disk apparatuses, such as a 1.8-inch magnetic disk apparatus. As micro magnetic disk apparatuses with a size of 1.8 inches or less have the size of a credit card, this type of magnetic disk apparatus can be detachably mounted in a computer and can be handled alone.
When this magnetic disk apparatus is handled alone unlike the one installed in a computer, it is susceptible to an external magnetic field and external electric noise, thus causing a read/write error. If the detachable magnetic disk apparatus is dropped on a floor or the like by accident, it would receive a great shock, which may damage the internal components.
This magnetic disk apparatus therefore requires good shock resistance, resistance to the external magnetic field and electric noise resistance. As means to reduce an external shock on the magnetic disk apparatus, it has been proposed to provide a shock absorber near the magnetic heads to prevent the head from being damaged by a shock (see Japanese Unexamined Patent Publication No. 168985/1991).
To protect magnetic disks from the internal magnetic field generated by the actuator in the magnetic disk apparatus, there has been proposed a shield structure such that the base on which the actuator is mounted should be made of a ferromagnetic material (see Japanese Unexamined Patent Publication No. 63077/1984).
While the shield structure of the conventional magnetic disk apparatus can shield an internal magnetic field, however, it cannot shield an external magnetic field. Accordingly, the read/write characteristic of the magnetic heads is impaired by the external magnetic field. In the vertical magnetic recording system, data on some magnetic disks may be erased by the external magnetic field. As a solution to those problems, the entire magnetic disk apparatus may be shielded by a metal plate or the like. This measure however causes the size of the apparatus to exceed the form factor size of the apparatus, increasing the space the apparatus needs for its installment. Further, since no measures have been taken against external electromagnetic noise as well as internal electromagnetic noise, those noises are likely to cause a data error.
Furthermore, in a contact type magnetic recording system, wear-out powder is produced by the contact, thus reducing the contact type magnetic recording performance.